1. Field of the Invention
This invention relates to wireless communications and, more particularly, to multipath processing for a wireless communications.
2. Description of Related Art
Traditional Code Division Multiple Access (CDMA) receivers depend on a spreading code design to mitigate the effects of multipath channels. Spreading sequences with good auto-correlation properties allow a RAKE receiver to detect and process individual multipaths before the individual multipaths are combined. In practice, ideal spreading codes are difficult to design. Further, as data rates and capacity demands increase for subsequent generation systems, simple RAKE-based detection will no longer be adequate.
Data rates of up to approximately 20 Mbps have been proposed for the High Speed Downlink Packet Access (HSDPA) mode of the Universal Mobile Telecommunications System (UMTS) third generation (3G) standard, using multiple antennae at the transmitter and receiver in up to a 4×4 configuration. With a chip rate of 3.84 Mcps, the spectral efficiency proposed is on the order of 5 bps/Hz, higher than conventional systems, but realizable for multiple antennae systems. Furthermore, with turbo codes used for channel coding, 3G systems may be capable of operating with high load, and hence, a modest signal to noise ratio (Ior/Ioc) budget (<10 dB).
With low signal levels and interference from other users, other transmit antennae, and multipaths, the initial detection and despreading phase are factors in overall performance. The 3G spreading codes are designed for orthogonality between users, or between multiple antenna of a given user at a basestation transmitter. These spreading codes do not have particularly good auto-correlation properties. As a result, much of the interference at the receiver is due to multipaths from the same transmit stream.
Equalizers have been designed to restore the chip pulse shape to re-orthogonalize the transmit signals, and combine multipath components of the one transmit stream in order to reduce total mean squared error (MSE). However, such structures have been described for single antenna configurations only.